The number of publications and patents covering the contact lens art is almost overwhelmingly extensive. Review articles have appeared periodically. For example, see those by Tighe, B. J., Brit Polymer J (September 1976) 71-77; Pedley, D. G. et al, Brit Polymer J 23:748-753.
Briefly, and in general, the first contact lenses were glass, but this was replaced in the late 1940's by the so-called "hard" contact lenses made mostly of polymethyl methacrylate The hard plastic lenses have excellent optical characteristics and good machining and polishing qualities, but are practically impermeable to oxygen due to their low moisture content, and they require a thick film of tear between themselves and the eye. A few other hard lens materials now available are somewhat more permeable to oxygen.
Another basic class, the flexible silicone lenses are oxygen permeable but are strongly hydrophobic which renders them uncomfortable and they are furthermore susceptible to discoloration due to absorbed lipids from tear fluid.
A third category, the soft hydrogel contact lenses, were first proposed in 1960 by Wichterle and Lim Nature (1960) 185:117. Various compositions based on lenses made of hydrophilic polymeric materials have been the subjects of a number of U.S. and foreign patents.
In general, the hydrogel lenses are addition polymers of acrylic or methyl acrylic acid ester derivatives with alcohols containing hydroxyl groups capable of conferring hydrophilicity. Commonly used materials include 2-hydroxyethyl methacrylate (HEMA), 2,3-dihydroxypropyl methacrylate (DHPM), hydroxyethyl acrylate (HEA); vinyl pyrrolidone (Vp), methacrylic acid (MAA); acrylic acid (AC), methyl methacrylate (MMA), glycerol methacrylate (GMA), and acrylamide (AA). The foregoing list is representative of materials which have been used either alone or in combination as the polymerizing material to form the hydrophilic lens. In addition, the polymers have been cross-linked by the inclusion of the diacrylic esters or dimethacrylic esters of ethylene glycol monomers and polymers, such as ethylene glycol diacrylate (EGDA) and ethylene glycol dimethacrylate (EGDMA) as well as the corresponding acrylates and dimethacrylates of polyethylene glycol (PEGDA and PEGDMA). These cross linkers are generally present in low amounts, approximately 0.1-5%.
In addition, soft but hydrophobic lenses have employed fluoroacrylates (FA) and fluoromethacrylates (FMA) either alone or as copolymers with more hydrophilic monomeric units.
Despite the increased comfort and convenience of soft contact lenses, which permit extended wear due to the ability of the lens to be compatible with the eye and permit the passage of oxygen to the cornea, problems have arisen because the high moisture content of soft lenses is also conducive to the absorption of proteins which in time discolor and obscure the transparency of the lens. The absorption of protein is serious not only because the lens is effectively fogged, but also because it provides a breeding ground for bacteria which can result in severe eye infection. The invention herein provides a solution to this problem by furnishing a range of compositions which, while retaining a high moisture content, substantially reduce protein absorption with its attendant problems.